Process for improving the dimensional stability of regenerated cellulose films



3,068,529 Patented Dec. 18, 1962 ice This invention relates to themanufacture of regenerated cellulose film. More particularly, thisinvention relates to a method for improving the dimensional stabilityand appearance of regenerated cellulose films.

The following block diagram illustrates the process of the presentinvention:

Softened Gel Regenerated Cellulose Film Drying While Applying DrawDrying While Permitting Relaxation Completing Drying While Applying DrawIt has been found that overwrapped packages exposed to high humiditydevelop a pattern of corrugations or ripples running in the machinedirection of coated regenerated cellulose film. This rip-ple patterndetracts so seriously from the appearance of the package that somepotential customers question the premium quality of these films. Theterm rippling has been given to the pattern of corrugation whichdevelops in cellophane used for overwrap on a package wrapped at low ornormal humidity and subsequently exposed to high humidity. Rippling isto be distinguished from puckering which is a localized sheet distortionusually associated with defects or nonuniformities in moistureproofcoatings. A puckered film will eventually flatten out as moistureequilibrium is reached in all parts of the film; rippling, however,persists as long as the film is exposed to the high humidity. Ripplinghas been a particularly serious problem on those films which have beenpromoted as a premium quality film giving improved appearance on thepackage.

Rippling of cellophane at high humidity is caused by simultaneousmachine direction shrinkage and transverse direction expansion of thefilm on an overwrap package when the moisture in the film is increased.There are variations in the degree of rippling and practically allcellophane films ripple to some extent.

It has been found that tension in the cast film during drying is themost import-ant single factor affecting rippling. Large variations inthe high humidity distortion have been seen among different castingmachines with different dryer types and different tension patterns.

While it is apparent that zero distortion in both directions would beideal, if the differential distortion be tween the distortion in themachine direction and that in the transverse direction was less than1.5%, substantially ripple-free films would be obtained. In any case,the distortion in either the machine direction or transverse directionalone must be less than 2%.

It is, therefore, an object of this invention to improve the dimensionalstability of regenerated cellulose films. A further object of thisinvention is to provide a method for producing substantially rippledreeregenerated cellulose films. A still further object is to provide amethod for producing such films while maintaining good film flatness andgood appearance.

These and other objects are accomplished by the present invention whichcomprises, in a process for improving the dimensional stability ofregenerated cellulose film, the improvement which consists in: (1)drying softened gel regenerated cellulose film to about to moisturewhile applying 2 to 4% draw, (2) further drying the said film to about25 to 60% moisture while permitting about 1.5 to 2.5% relaxation, and(3) completing said drying to about 3.0 to 7.5% moisture while applyingfrom about 0.5 to 1.0% draw.

In a preferred embodiment, the drying in step (2) will be conducted toabout 25 to 35% moisture.

The term percent moisture as used herein refers to the percentage ofWater present based on the dry weight of cellulose over and above thedry weight of the cellulose itself (e.g., 50% moisture means for everygrams of wet cellulose, 50 grams of water and 100 grams of dry celluloseare present).

The term percent draw as used herein means the percentage extension oflength of the film caused by stretching in the machine direction. Theterm applying draw means the step of extending the length of the film inthe machine direction by a given percentage of its original length.

The term percent relaxation is the opposite of the term percent draw andrefers to the percentage decrease in length of the film caused byshrinking in the machine direction. The term permitting relaxationrefers to the step of decreasing the length of the film in the machinedirection by a given percentage of its length by taking advantage of thetendency of the moist film to shrink. Specific means for accomplishingdraw and relaxation of the film will appear hereinafter.

The term soften gel regenerated cellulose refers to regeneratedcellulose in a gel condition containing about 3.0 to about 3.5 times theweight of dry cellulose as water. Further clarification of this termwill appear hereinafter.

The initial distortion of cellophane (increase or decrease in length) inpassing from 35% humidity to 81% humidity has been the most importantcriteria yet devised for predicting ripping.

It has been found that when softened gel re enerated cellulose film wasdried from about 100 to 120% moisture with 2 to 4% draw, and furtherdried to 25 to 60% moisture with 1.5 to 2.5% relaxation, and the dryingcompleted to 3.0 to 7.5% moisture with 0.5 to 1.0% draw, thedifferential between the transverse direction distortion and machinedirection distortion was not greater than about 1.5%. Films produced bythe described process were ripple-free when exposed to high humidity(i.e., 81% relative humidity), and had good appearance and good filmflatness.

The optimum increase or decrease in draw or relaxation, accomplished bydifferential peripheral speeds of dryer rolls, differ somewhat fordifierent thickness of film, and for films with different degrees ofplasticization. For this reason it is desirable to be able to vary thedifference between peripheral speeds before and after the points ofcritical film moisture content.

The process of the present invention is generally applicable to thedrying of continuous Webs of any wet cast cellulosic film. However,because of the commercial importance of regenerated cellulose filmderived from viscose, such film constitutes the preferred material fortreatment in accordance with theprinciples of the present invention.

The commercial scale method of continuously manufacturing regeneratedcellulose film from an aqueous solution of sodium cellulose xanthate(viscose) is disclosed in US. Patents 1,548,864 and 1,601,289 toBrandenberg-er. In this process, viscose is forced through an elongatedorifice in the form of a sheet into a coagulating bath to form acoherent web, which freshly coagulated film is then promptlyregenerated, washed, desulfured, bleached, softened and dried. The driedfilm is wound into rolls for convenience in handling, storing, andfurther processing. Up through the softening stage the web ofregenerated cellulose remains in a gel condition (i. e., completelysaturated with aqueous solution). Since gel regenerated cellulose filmcontains about 3.0 to 3.5

film obtained). To prevent excessive loss in area, the

formation of surface irregularities, pucker's, wrinkles and the lik, theimpairment of transparency, it is customary men; the Webby passing itthrough a series of heated rolls, such as described in Herdon, US.Patent 2,000,-

l79; C hylinski, US. Patent 2,141,277; Stevens, US. Patent 2,746,166 andBritton et al., U.S. Patent 2,746,167. The pr'esefit invention isconcerned with the drying stage of the above-described process ofmanufacture.

1 Wet, softened, gel regenerated cellulose film, after leaving thesoftener tank, passes between squeeze rolls "or scraping devices whichremove excess softener bath and enters the first section of the dryerand contains approximately 300 to 350% water, based on the weight of drycellulose. In the first stage of the dryer the film passes over a seriesof conventional dryer rolls operated at a peripheral speed greater thanthe rolls in the wetend of the casting machine which will maintain theweb under 2 to 4% draw. The dryer rolls throughout the dryer areuniformly heated and heated air is circulated across the web so as tomaintain the web at substantially uniform drying temperature to ensure asymmetrical transverse moisture distribution profile. When the moisturecontent of the film has been reduced to approximately 100 to 120%moisture based on the weight of dry cellulose, the web is passed over asecond series of conventional dryer rolls driven at a peripheral speed1.5 to 2.5% lower than that of the preceding rolls to effect a decreasein, or a relaxation of, the tension of web. In this stage of the dryer,the moisture content of the film is reduced to to 60% after which thefilm is passed through an additional series of conventional dryerrollsdrawn at a peripheral speed of from 0.5 to 1.0% greater than that of therolls in the previous stage to exert 0.5 to 1.0% draw on the film. Inthis stage, the moisture content of the film is reduced to about 3.0 to7.5% moisture based on the weight of dry cellulose. Thus, with thedrying completed, the film leaves the dryer and is collected as a millroll.

The table below indicates the various tension patterns used in eightdifferent examples with the resulting distortions having been producedby subjecting the finally dried regenerated cellulose films to thedistortion test which will be described below. In the following table MDindicates the distortion in millimeters in the machine direction, and TDindicates the distortion in millimeters in the transverse directionafter the samples have been subjected to the distortion test.

DISTORTION TEST Samples of regenerated cellulose film (about 0.88 milafter drying) dried under the conditions indicated in Examples 1 to 8inclusive, below, were coated on both sides with a vinylidene chloridecopolymer from a sol vent solution, properly dried to evolve solvent,and then conditioned at F. and 35% relative humidity for 72 hours.(After coating, the films were about 1 mil in thickness.) After thisconditioning, strips of film 400 millimeters in one direction by oneinch in the other direction were cut approximately out of the center ofa full cast width of the coated sheet of regenerated cellulose. Sampleswere cut such that some samples had the long direction parallel to thetransverse direction and some parallel to the machine direction. Each ofthe film samples was supported against a frame having the approximatedimensions of each film sample, and the film sample was fixed at the topof the frame so that its long dimension was running vertically when theframe was hung in a conditioning chamber. A 20 gram weight was clampedat the bottom of each film. The frames to which each film sample wasafiixed at one end thereof were hung in a room maintained at 81%relative humidity and 85 F. The change in dimensions under theconditions of 81% relative humidity was recorded as change in length (inmillimeters) after 72 hours. The corresponding values are recorded inthe following table under MD and TD.

The percent distortion in the machine direction (MD) and transversedirection (TD) is calculated by the follow- The differential distortion(expressed as percent) is calculated by the following:

400 mm. X

=Percent Differential Distortion In the following tables when the signin front of MD (mm.) and TD (mm.) is plus it means that the film isexpanded in the test. When the sign is minus it means that the filmshrank in the test. It should be emphasized, however, that even thoughthe sign of TD (mm.) may be minus it is always used as a plus value inthe above formulation when calculating percent differential distortion.The calculation is carried out in this manner because it is desired, ifthere is any change in dimension at all in the film during the test,that the change in dimension be an increase (plus) to avoid anysemblance of rippling, it being understood, as mentioned before, thatthe percent distortion in either the MD or TD alone must be less than2%.

Examples 1 and 2 below illustrate operation within the scope of thisinvention:

For comparison, the following Examples 3 to 8 are given of operationoutside the scope of this invention:

Moisture Tension MD TD Difierential Ex. content pattern (mm.) (mm)distortion c (percent) 3..-- Gel to 100% 2.0% draw 7.1 2.1 2.3 100% to30% 2.0% draw"--. 30% to 5% 2.0% dra\'\" 4.. Gel to 100%..-" 2.0% draw+1.3 +8.2 1.. 7 10 100% to 5% 1.5% relax".-.

5 Gel to 30% 1.57 relax. +1.1 +7.3 1. 5

30% to 5% 2.0%; draw..-"

6 Gel to 100% 2.3% draw +2.75 +8.75 1.5

100% to 30%--. 4.6% relax.-- 5 30% to 5% 1.6% draw 7 Gel to 100%..- 3.9%drawn-" +1.2 +8. 65 1.96

100% to 30%-.." 1.4% relax. 30% to 5% 0.29% draw 8. Gel to 100% 4.9%draw- +0. 5 +12. 75 3. 06

100% to 30%.--- 4.7% relax. to 5% 4.0% draw.

Norm-The underlined figures indicate distortions and differentialdistortions (percent) above allowable limits.

Below are listed additional comments on the films processed inaccordance with the above examples:

(1) The differential distortion was about 1.25%.

Comments The film did not ripple and had good film flatness andappearance.

(2) The difierential distortion was about 1.05%.

The

and excessive edge droop. (7) The transverse direction distortion wasgreater than and excessive edge droop. (8) The transverse directiondistortion was greater than and excessive edge droop.

The dif- The film rippled, had poor appearance The film rippled, hadfair appearance The film rippled, had poor appearance The above examplesillustrate the criticality of the limitations in the invention byshowing that operation out side the scope thereof yields films which arefor one or more reasons unacceptable.

Flatness of a film is a characteristic of continuous films which is animportant determining factor in their running abilities on converter andprocessing equipment. An absolutely fiat sheet, having no droop, is thegoal of all continuous film manufacturers. Flatness is determined bystretching a film longitudinally across two carefully aligned,horizontal parallel supporting rolls or bars 15 feet apart. The filmends are rigidly held and tension is applied longitudinally to the filmover the unsupported span of 15 feet. The contour of a transversesection of the film at approximately the center of an unsupported spanis observed and the major departures from the straight, horizontal lineare measured in inches. Experience has shown that a lack of flatnessand, particularly, the presence of a slack longitudinal lane between twosupporting lanes, called center droop, is extremely detrimental tosatisfactory operation in bag making machines, wrapping machinery,printing presses, etc. Drooping of the film edges, depending upon theseverity, is also undesirable. Film flatness, as measured by droop, isconsidered unsatisfactory for use on such automatic equipment, if theedge droop exceeds one-quarter inch at 1.4 lbs. of tension per inch offilm width. The films processed within the scope of this inventionexhibited satisfactory flatness when measured by the above test.

This invention provides for the production of a trade acceptable filmfrom the standpoint of rippling, while maintaining good appearance andthe desired film flatness.

What is claimed is:

1. In a process for improving the dimensional stability 0t regeneratedcellulose film, the improvement which consists in: (1) drying softenedgel regenerated cellulose film to about to moisture while applying 2 to4% draw, (2) further drying the said film to about 25 to 60% moistureWhile permitting about 1.5 to 2.5% relaxation and (3) completing saiddrying to about 3.0 to 7.5% moisture While applying about 0.5 to 1.0%draw.

2. Process according to claim 1 wherein the moisture in step (2) is fromabout 25 to 35% moisture.

References Cited in the file of this patent UNITED STATES PATENTS2,115,132 Alles et a1 Apr. 26, 1938 2,517,570 Irons Aug. 8, 19502,955,345 Howe Oct. 11, 1960

1. IN A PROCESS FOR IMPROVING THE DIMENSIONAL STABILITY OF REGENERATEDCELLULOSE FILM, THE IMPROVEMENT WHICH CONSISTS IN: (1) DRYING SOFTENEDGEL REGENERATED CELLULOSE FILM TO ABOUT 100 TO 120% MOISTURE WHILEAPPLYING 2 TO 4% DRAW, (2) FURTHER DRYING THE SAID FILM TO ABOUT 2.5 TO60% MOISTURE WHILE PERMITTING ABOUT 1.5 TO 2.5% RELAXATION AND (3)COMPLETING SAID DRYING TO ABOUT 3.0 TO 7.5% MOISTURE WHILE APPLYINGABOUT 0.5 TO 1.0% DRAW.